Communication among immunocompetent cells is crucially required for the controlled expression of immunity. Work related to this project has been designed to investigate a form of cellular communication termed the syngeneic or autologous mixed lymphocyte reaction. This phenomenon represents proliferation occurring among T cells when they are appropriately co-cultured with autologous non-T cells. Utilizing a series of positive and negative selection procedures as well as monoclonal antibodies, it can be demonstrated that the autologous mixed lymphocyte reaction is heterogeneous and actually consists of interactions occurring among two distinct stimulator and responder cells. Most importantly, at least a portion of the autologous mixed lymphocyte reaction, that occurring between T cells and macrophages, reflects interactions occurring between antigen reactive T cells and antigen presenting macrophages. Thus, at least part of the autologous mixed lymphocyte reaction reflects interactions occurring among immunologically relevant cells and should serve as a model useful for the investigation of biochemical and cell membrane events involved in this interaction.